Weather Basics: Clouds and Precipitation

Clouds and Precipitation

Clouds consist of tiny water droplets—or ice crystals in colder parts of clouds—that are light enough to remain suspended in the air.

How Clouds Form

The dew point is the temperature air must reach, at constant pressure, to become saturated. At saturation, relative humidity is 100%. Further cooling can cause water vapor to condense.

Air can be lifted and cooled to form clouds in three main ways:

• Heating: The Sun warms the ground, causing warm air to rise.
• Frontal lifting: Weather fronts force air upward.
• Orographic lifting: Mountains or other terrain force air upward.

Cloud Types and Levels

• Cumulus: Puffy clouds with flat bases, usually below 6,500 feet. They often occur in fair weather but can grow into cumulonimbus clouds when the atmosphere is unstable.
• Stratus: Low, broad gray layers formed as large air masses are lifted. They may bring drizzle, light rain, or light snow. Fog is stratus at the ground.
• Cirrus: High, wispy clouds, generally above 20,000 feet, made of ice crystals. They can signal approaching thunderstorms. Aircraft contrails are a type of cirrus cloud.

Clouds are also grouped by altitude:

• Low: Bases below 6,500 feet
• Middle: Bases between 6,500 and 23,000 feet
• High: Generally above 20,000 feet

The video also describes mammatus clouds as pouch-like features beneath storm clouds, orographic clouds as clouds associated with airflow over terrain, and altocumulus castellanus (ACCAS) as a sign of instability aloft that can precede afternoon thunderstorms.

How Precipitation Forms

Precipitation can be liquid, freezing, or frozen. The video describes two formation processes:

• Collision and coalescence: Cloud droplets collide and combine until they are too heavy to stay aloft, then fall as precipitation. This process commonly produces warm rain.
• Ice-crystal process: Ice crystals and supercooled water droplets coexist in a cloud. Water vapor deposits onto the ice crystals, which grow, fall, and may collide with other particles. The video describes this as the dominant precipitation-forming process across the United States.

The temperature profile between the cloud and the ground determines what reaches the surface:

• Rain: Liquid droplets reach the ground without freezing.
• Snow: Ice crystals remain frozen all the way down.
• Sleet: Snow partially melts in a warm layer, then refreezes in a colder layer below.
• Freezing rain: Snow melts completely in a sufficiently warm layer, then freezes on contact with a subfreezing surface.
• Drizzle: Small liquid droplets, usually from low stratus clouds.
• Freezing drizzle: Drizzle droplets freeze when they contact a surface at or below freezing.
• Hail: Listed as a frozen precipitation type, though its formation is not explained in detail.

Other Facts

The video states that precipitation covers about 2–5% of Earth at any given time, raindrops fall at roughly 7–18 mph, and a typical raindrop is about 1–2 millimeters across. It also explains that falling raindrops become flattened rather than teardrop-shaped, and that very large drops can break apart.

Researchers and Sources

No individual researchers or external references are named in the subtitles.

Source/presenter: National Weather Service Weather Forecast Office La Crosse, Wisconsin (NWSLaCrosseWI).